Biological causes of an anxiety disorder include, but are not limited to,
a) genetic influences and chemical imbalances in the brain
b) anxiety sensitivity and pairing neutral stimuli to frightening stimuli
c) our evolutionary history of historically frightening stimuli
d) our capacity of observing, retaining, and replicating behavior

A) genetic influences and chemical imbalances in the brain

Explanation:

An anxiety disorder is a psychiatric disorder where the individual experiences an excessive and irrational worry in everyday situations. Generalized anxiety disorder, phobias, panic attacks, etc are examples of anxiety disorders.

Anxiety does not have a specific cause, it can be the result of different factors like changes in the brain's chemistry, environmental stress or even genetics.

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the answer is:

carbon dioxide